Engine stop-start -vs- idling in traffic: How much can you save?

How much fuel and therefore money can you actually save using engine stop-start systems? How much use do you suppose stop-start needs in order to barely break even? Here’s the answer…

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I get three or four of these comments a week, from dickheads just like this:

"Watch Engineering Explained channels video that runs the actual math on this topic.  You’ll find actual science completely contradicts the s*** you hear spewing from this dissertation."

- NorthernChev 

Jason does a great job with his channel, but this report used some out-of-date data to draw an erroneous conclusion unhelpful to consumers…

Engineering Explained. I really like Jason Fenske’s channel - Engineering Explained. I’ve watched dozens of his reports. He’s very successful, and he deserves to be. He tackles some hardcore topics and makes them pretty digestible for the mainstream.

But on the 5th of September last year, Jason put up a video called ‘Americans have no idea how much fuel idling uses’. 

Jason referenced a 14 year old study in the SAE for most of the data, and the mathematics is absolutely on the money - but I respectfully believe he’s sending the wrong overall message. Profoundly wrong. And here’s why.

Basically, it would be easy to watch that report on Engineering Explained and infer that it’s a good idea to shut your engine down if you look like being stopped for more than seven seconds. 

Because you are (quote-unquote) “wasting fuel” beyond that point, according to Mr Fenske. Which is synonymous in most people’s minds with ‘wasting money’.

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Fired up

But I’d suggest this conclusion a viewer might draw - about seven seconds being the ‘go/no-go’ threshhold for shutting down - is simply bullshit - using Professor Harry G Frankfurt’s definition of that term. 

And this is not my opinion - I’m using research and data from Argonne National Laboratories in Illinois - one of the US’s premier engineering research centres.

You’ve heard of the Manhattan Project right? Big win for our team in August 1945. Argonne did that.

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In April 2015, the dudes at Argonne published a report with the catchy title ‘Stop and Restart Effects on Modern Vehicle Starting System Components - Longevity and Economic Factors’. Miracle cure for insomnia, right there, for many people. But not me, or you.

Scientifically robust. They conscripted the technical input from six automakers, plus Denso and Johnson Controls, the SAE and Oak Ridge National Laboratory. It’s kinda the ‘works burger’ of engineering intellectual grunt on this issue.

It’s 20 pages of heavy going - a full economic analysis of the potential cost implications) of choosing to manually shut down your engine in traffic.

“...a minimum shutdown duration of approximately one minute for six or fewer additional starts per day results in economic savings due to a reduction in idling fuel cost.”

- Argonne National Lab.

This is a hardcore academic analysis which basically took all the technical factors into account - like starter and battery design life - and corrected the fuel saving for fuel used on restart, the wear effects on starter motors, the battery life implications of more frequent restarts and additional charge depletion, etc.

And they did it for a projected 10-year period of the vehicle’s life.

What they found was that if you shut down and restart often, like 12-20 times a day, and if the shut down duration is short, like, less than 2-3 minutes, then doing this will actually cost you money.

If you shut down fewer times - like, six or eight times a day - for more than about five minutes per shutdown - you’ll save money.

But in either case, the amount is completely trivial, in the context of household budgets. Maximum possible saving for a two-litre engine: $400 (US dollars) over 10 years. To do that, you need to be shut down for eight or nine minutes at a time. That’s 11 cents a day (US).

Worst case, two-litre engine: 18-20 re-starts daily, each for a minute or two … that’s going to cost you an additional $600 (US dollars) in accelerated battery death and starter wear. Call it 16 cents a day (US).

That’s a loose definition of ‘beneficial’, in my view. I’d suggest that there’s no other domain in human life where people would devote any attention span at all to an activity that will impact their lives between +11 and -16 cents per day. The concept is absurd.

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